Housing Capacity Is Not Housing Supply: A Gate-Based Framework for Measuring Conversion

Abstract Housing targets are frequently expressed through quantities of land, planning permissions, vacant properties, public assets, financed projects or planned construction. These indicators are informative, but they do not measure the same phenomenon. This paper proposes a gate-based framework for distinguishing nominal housing capacity, conversion capacity and realised housing supply. The framework emerged from the SCI Portugal Housing Market Capacity Study and was examined against two complementary evidence layers: a formal structured scoping evidence review (PRISMA-S1) comprising 46 independent eligible studies, with bounded search completeness; and a targeted cross-case validation set used to examine selected mechanisms involving planning reform, construction capacity, infrastructure and finance. The evidence is consistent with housing delivery behaving as a serial conversion process: increasing capacity or relaxing a constraint at one stage can increase output, but does not guarantee additional delivered housing when downstream or interacting constraints remain binding. Applied to the Portuguese H-300 scenario of 300,000 dwellings over ten years, the framework does not establish feasibility or infeasibility. Demonstrated quantitative capacity remains not determinable because no non-duplicated quantity has been shown to traverse the full conversion chain at the required scale and within the relevant timeframe. The paper therefore shifts the policy question from how much potential housing exists to how much housing can demonstrably traverse the relevant conversion chain and become available for use.